1. Field of the Invention
This invention relates to a positionable antenna, and in particular to a positionable narrow beam microwave antenna.
2. Description Relative to the Prior Art
The microwave antenna having a linear array of electromagnetic radiating slots is known in the art. The slots serve as dipoles, generally spaced at distances λ/2 apart, (λ=wavelength), and generate a highly directional single lobe radiation beam due to the mutual interference of the dipole outputs. Typically, slots arranged in a square array generate a beam that is narrow in both the azimuthal and elevational planes. An important feature of narrow beam antenna performance depends upon the accurate positioning of the beam in a selected direction to maximize signal transmission or reception. In the prior art, it is known that antenna servo control signals for antenna positioning in the azimuthal and in elevational directions may be derived by “dithering” the beam in the two orthogonal directions. “Dithering” consists in cyclically shifting the antenna's beam from its aimed position to alternately derive “left-right” and “up-down” received signal responses from the displaced beam whose amplitudes are compared and converted into servo control signals for positioning the antenna. Since it uses the r.f. information signal itself, this method generally reduces the signal-to-noise ratio of the antenna, as “dithering” requires the continuous sweeping of the antenna pattern through off axis directions.
It is also known in the art that an appropriately shaped dielectric rod antenna, which may be a ferrite, exhibits directional radiation characteristics when end-fed with microwave electromagnetic excitation. Dielectric rods are particularly effective when configured as elements of a microwave antenna array, and the use of dielectric rods as such directional antenna elements is described in detail in the article entitled “Ferrod Radiator System” by F. Reggia, E. G. Spencer, R. D. Hatcher, J. E. Tompkins, Proceedings of the IRE, 45 (1957) #3.
Seen in FIG. 1, a dielectric rod radiator, 10, has a dielectric shaft, 12, tapered to suppress side lobes, and to match its end impedance to a free space impedance of 377 ohms. This rod, fabricated from a dielectric material which may be a ferrite, has a permittivity of, e.g., 10, and a gain of 35 compared to a standard horn radiator. End fed with 1.5 cm electromagnetic energy, the directional radiation pattern, 14, of such a dielectric rod is shown in FIG. 2. The angle Θ is measured in the plane formed by the axis of the rod and a line perpendicular to the axis, the 0 of angle Θ being in the direction of the line perpendicular to the rod axis. The peak direction of the radiation pattern is along the rod axis, and hence occurs at Θ=π/2. It will also be noted that due to reciprocity the pattern of an antenna is the same both for transmission and reception.